The rate of a person's thermal exchange with the environment is dependent upon a number of factors, such as activity level, the nature and condition of the environment, and the characteristics of the clothing having contact with the wearer's skin. The transfer of body heat from the skin through clothing or footwear to the environment takes place by conduction, convection, radiation and evaporation, the physiological details and mechanisms of which are generally well known.
In footwear, conduction or convection from the foot through the shoe to the environment is the main mode of heat loss from the foot to the outer surface of a shoe or boot. Thermal energy is principally transferred by conduction through the footwear, and is thereafter transferred by conduction or convection as well as radiation from the sole to the environment.
Given the propensity for the extremities and particularly the feet to easily become cold, it is logical for efforts to have been made to improve the insulating capacity of shoe soles and/or insoles. U.S. Pat. No. 4,658,515 to Oatman entitled "Heat Insulating Insert For Footwear" discloses a heat retention insole for shoes comprising a laminated structure incorporating a flexible plastic bottom sheet, a similar plastic upper sheet, a space sheet formed primarily of a foamed organic plastic material. The spacer sheet includes large area apertures defined by bars extending transversely between portions of a parametrial band portion. These large area apertures are loosely filled with particles of insulating plastic materials (such as hollow polystyrene or polyethylene beads) which also serve to maintain the top and bottom sheets in a spaced relationship even when subjected to foot pressure. The outer surfaces of the insole are preferably provided with a heat reflecting coating or film to minimize the loss of heat from the foot by radiation and reflect any such radiated heat back in to the foot of the wearer. While somewhat effective, this insole relies on trapped air within cellular spaces to retard the flow of heat. Compression of the insulating material reduces the volume of the trapped air space which, in turn, increases thermal conductivity and increases the rate of heat loss through the footwear.
Another example of an insulating insole is disclosed in U.S. Pat. No. 4,055,699 to Hsiung which teaches a multi- layer insole that is adapted for insertion into an article of footwear to insulate the foot from the cold developed in the sole and sock lining, from walking on a cold surface, the insole being thin enough to fit within the footwear without requiring an increase in footwear size. The insole comprises four layers, including from top to bottom, a top layer of soft fabric for comfortable contact with the plantar surface of the foot or sock of the wearer, a second layer of thermoplastic foam cushioning material, a third layer which is more effective than the remaining layers in insulating the foot against cold, and a fourth layer that is an aluminized polymeric material of paper thinness. Again, compression reduces efficacy.
Still another attempt at maintaining the warmth of the foot is found in U.S. Pat. No. 4,331,731 to Seike et al. which discloses an exothermic body for use as a shoe insole. The insole includes a foamed plastic sheet having cells therein and an exothermic agent such as a mixture of iron, saline solution, activated carbon and woodmeal or pulp powder. The foamed sheet is covered by an air-permeable, thin film such as a thick perforated plastic film. A cushion material is provided on at least one surface of the foamed plastic sheet covered with the air-permeable thin film and the foamed plastic sheet together with the cushion material covered by an air permeable film cover. This device is not rechargeable and is, therefore, an expensive and temporary solution to the problem.
It is, therefore, an object of the present invention to provide a shoe insole having enhanced ability to insulate the foot from hot or cold.
Another object of the present invention is to provide a shoe insole that is compatible with existing footwear and which may be used without the necessity of the user purchasing footwear larger than that normally worn.
Yet another object of the present invention is to provide a shoe insole with enhanced insulating properties that also has a high degree of structural integrity.
A still further object of the present invention is to provide a shoe insole having enhanced thermal storage properties that is reusable.
A related object of the present invention is to provide a shoe insole with limited compressibility and which, therefore, enhances thermal management of the foot.